Fuel cells are receiving increasing attention as a viable energy-alternative. In general, fuel cells convert electrochemical energy into electrical energy in an environmentally clean and efficient manner. Fuel cells are contemplated as potential energy sources for everything from small electronics to cars and homes. In order to meet different energy requirements, there are a number of different types of fuel cells in existence today, each with varying chemistries, requirements, and uses.
In a conventional fuel cell, the fuel and oxidant flow in separate streams, kept apart by an ion conducting membrane that divides the cell into discreet anode and cathode chambers. The single-cells are stacked in series electric connection using bipolar flow field plates that provide most of the stack weight and volume. The membrane and bipolar plates contribute respectively 15-68% and 10-25% to the stack cost, depending on the intended application and stack design. By comparison, in a mixed reactant fuel cell (MRFC), a mixture of fuel and oxidant flows through the cell as a single stream. Although the mixed reactant concept is generally thought to flout reaction thermodynamics, it is possible to contrive a practical mixed-reactant system based on the following three kinetic effects: a) avoiding spontaneous thermochemical reaction between the fuel and oxidant that may occur in the bulk reactant mixture or on catalyst surfaces, b) providing intrinsic kinetic selectivity of the anode and/or cathode electrocatalysts to suppress mixed-potentials of electrodes; and c) promoting selectivity of the electrodes for mass transfer of the fuel and oxidant respectively to the anode and cathode. Previously described MRFCs have suffered from an inability to optimize these mechanisms, leading to thermochemical electrochemical, and/or mass transfer defects that lower the cell voltage, increase fuel consumption, and decrease the energy efficiency of the MRFC.
A specific example of a MRFC, is a direct borohydride-oxygen fuel cell (DBFC), which uses borohydride as a fuel. Borohydrides (such as NaBH4) and their various derivatives are intensely researched for alternative energy related applications as either hydrogen storage compounds or as ‘electrochemical fuels’ including their use in fuel cells. In DBFCs, borohydride is supplied directly to the anode as an alkaline solution. Compared to other direct liquid fuel cells (i.e., methanol, ethanol, or formic acid), DBFCs possess two important advantages: i) higher theoretical energy density (9.3 kWh kg-1 of NaBH4); and ii) the inherent absence of carbon in the fuel. The inherent absence of carbon implies that a DBFC can be operated as a zero carbon emission device. Furthermore, because the DBFC does not produce CO, a well-known intermediate formed during electro-oxidation of fuels such as methanol, ethanol and formic acid and is known to act as catalytic poison), alternative anode options are available.
With respect to the DBFC design, most of the published literature employs the conventional dual-chamber proton exchange membrane (PEM) technology in a single-cell configuration. While this set-up is adequate for laboratory scale catalyst research purposes, the plate-and-frame PEM fuel cell stack design, imported unchanged from the hydrogen-oxygen fuel cell research, poses several challenges for the scale-up and stack design of alkaline DBFC. Some of these challenges are: PEM durability in the concentrated alkaline electrolyte; need for both gas-tight and liquid-tight sealing; use of heavy and expensive bipolar flow-field plates that must withstand the concentrated alkaline solution; and need for fairly complex stack manifolds to assure uniform distribution of the alkaline borohydride solution to each anode in the stack with low pressure drop. There have been very few publications addressing any of these very important issues concerning the DBFC technology and its scale-up. Yang et al. replaced the PEM with a hydrophilic polymer and reported a single-cell power density of 663 mW cm-2 using a Co-based anode. See, e.g., X. Yang, Y. Liu, S. Li, X. Wei, L. Wang, Y. Chen, Scientific reports 2012, 2, 567, which is incorporated herein by reference.
From the point of view of oxygen electroreduction cathode catalyst, the alkaline electrolyte needed to stabilize NaBH4 offers the possibility of using non-platinum cathode catalysts. It has been well-documented in the literature that the oxygen reduction reaction (ORR) in alkaline electrolytes is catalyzed by non-platinum group (non-PGM) catalysts as well such as Ag, MnO2 and various activated and doped carbon. The number of electrons exchanged in ORR per oxygen molecule is mainly dependent on the electrocatalyst and on the electrode potential and it varies between two and four. Carbon black electrodes in alkaline media catalyze only a two-electron transfer leading to HO2−. However, for alkaline fuel cells, the four-electron pathway is more efficient and thus preferred. Among the platinum-free ORR electrocatalysts capable of catalyzing a four-electron transfer, catalysts formed from one or more transition metals, nitrogen, and carbon, (M-N—Cs), are attractive candidates due to their high surface area, high activity, and low cost. The conventional synthesis of M-N—C catalysts involves various precursor deposition steps onto the high surface area carbons, resulting in a catalyst formed from a combination of the active material with an inert carbon matrix. Unfortunately, the presence of the carbon matrix substantially decreases the density of active sites for the four-electron pathway, somewhat neutralizing the advantage gained by using an M-N—C catalyst.
Accordingly, while DBFCs appear to be promising avenues for cost-effective zero carbon emission energy production, several challenges must be addressed including both the design over the fuel cell itself and optimization of a catalyst for use in a DBFC.
Accordingly, while MRFCs, including, but not limited to, DBFCs have the capacity for lower capital costs and higher power densities, there are several design issues that must be addressed before they are a commercially viable energy alternative.